This invention relates in general to monopulse, fanbeam search radars and, more particularly, to a means of obtaining accurate azimuth and elevation measurements in such radars in the presence of surface-induced multipath returns and ship's roll and pitch.
The presence of land-or sea-surface-induced multipath returns complicates the search radar detection problem. Target height measurements without multipath returns can be achieved (at least in all but the lowest beam) with a three-dimensional pencil-beam radar, but such radars often have difficulty scanning through the search volume in short times. Because few pulses strike a target, multiple beams and large antennas are often required, resulting in expensive systems. The two-dimensional radar, having a fan beam which covers all elevations simultaneously, is usually less expensive, has a smaller antenna, and has more pulses on target than the three-dimensional radar, but its performance suffers considerably due to surface-induced multipath signals.
The receipt of multipath signals occurs when the energy reflected from a target is received both directly from the target and indirectly from a reflective surface such as the sea. As a result of the multipath signals, two targets (the actual target and an image target) are at the same range and within the radar beam at the same time. The return signal seen by the monopulse radar system is the sum of the direct signal and the reflected signal. As target elevation changes, the amplitude of the return signal fluctuates because the phase of the indirect signal varies relative to the phase of the direct signal. This circumstance causes target fading in a fan-beam radar.
Multipath returns may also cause poor angle measurements in elevation and azimuth. Since a monopulse radar normally obtains the elevation angle of a target by measuring the signal in two beam patterns (i.e., the elevation angle is related to the relative strength of the return signal in each beam), the multipath signals will cause severe errors in elevation angle measurements. In addition, when the radar platform experiences level and cross-level rotations, as is the case in shipboard systems that experience roll and pitch, the cross-level rotation and the multipath will interact to give severe errors in azimuth measurements.